Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves and clothes dryers. A ceramic igniter typically has a hair-pin shape which contains conductive end portions and a highly resistive middle portion. When the igniter ends are connected to electrified leads, the highly resistive portion (or "hot zone") rises in temperature. Some of these igniters must meet the following requirements set by the appliance and heating industries to anticipate variations in line voltage:
______________________________________ Time to design temperature &lt;5 sec Minimum temperature at 85% of design voltage 1100.degree. C. Design temperature at 100% of design voltage 1350.degree. C. Maximum temperature at 110% of design voltage 1500.degree. C. Hot-zone Length &lt;1.5" Power (W) 65-100. ______________________________________
U.S. Pat. No. 5,085,804 ("the '804 patent") along with companion U.S. Pat. No. 5,405,237 disclose compositions suitable for a hot zone of a ceramic igniter, the hot zone comprising:
(a) between 5 and 50 v/o MoSi.sub.2, and PA1 (b) between 50 and 95 v/o of a material selected from the group consisting of silicon carbide, silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium aluminate, silicon aluminum oxynitride, and mixtures thereof. According to the '804 patent, these compositions provide the proper speed, room temperature resistivity and high temperature resistivity required for attaining the above-noted requirements without constraining the shape of the igniter. PA1 a) a pair of electrically conductive portions, each portion having a first end, PA1 b) a resistive hot zone disposed between and in electrical connection with each of the first ends of the electrically conductive portions, the hot zone having an electrical path length of less than 0.5 cm, and PA1 c) an electrically non-conductive heat sink material contacting the hot zone. PA1 a) providing a ceramic igniter comprising: PA1 b) applying a voltage of between 3 V and 60 V between the conductive ends of the igniter to produce an inrush current and a steady state current such that the ratio of the steady state current to the inrush current is at least 35% (preferably at least 50%), and raising the temperature of the hot zone to about 1350.degree. C. in less than 3 seconds (preferably less than 2 seconds).
One conventional igniter, the Mini-Igniter.TM., available from the Norton Company of Milford, N.H., uses a hot zone composition from the '804 patent which comprises aluminum nitride ("AlN"), molybdenum disilicide ("MoSi.sub.2 "), and silicon carbide ("SiC") and a total hot zone length of between about 1.5 cm (for 12 V applications) and 6 cm (for 120 V applications). Although the Mini-Igniter.TM. performs well in many applications, its speed (i.e., the time it takes to heat up from room temperature to the 1350.degree. C. design temperature) is typically between 3 and 5 seconds (for 24 V to 120 V applications). It is believed the applicability of these igniters could be greatly expanded if their speed could be decreased below 3 seconds.
Attempts have been made to increase the speed of these igniters. For example, Washburn and Voeller, "Low Power Gas Ignition Device, presented in the Proceedings of the 1988 International Appliance Technical Conference--Europe" (1988), pp. 134-149, discloses achieving speeds as low as 1.5 seconds by reducing the mass of the hot zone to about 0.07 to 0.08 grams (i.e., a length of about 1.0 cm to 1.3 cm). However, it is believed these igniters would be very susceptible to blowout caused by convective cooling. Willkens et al. "High Voltage Miniature Igniter Development", International Appliance Technical Conference, Madison, Wis. (1994) advise designing the length of the hot zone to be at least 0.7 inches (1.8 cm) for a 120 V igniter. The '804 patent also advises providing a hot zone length of at least 0.2 inches (or about 0.5 cm) as a practical minimum limit.
In addition, these igniters generally experience a very high in-rush current (i.e, a current of about 10 amperes in the first millisecond) before settling down to a conventional 2 to 3 ampere current. Since any transformer designed for use with these igniters must be designed to accept this initial high current, these igniters must be paired with a transformer capable of receiving higher power instead of the less costly transformer rated for a lower power.
Simply lowering the resistivity of the hot zone composition (by increasing its conductive MoSi.sub.2 content) has been considered as a method of increasing the speed of the igniter. However, it was found that doing so increases the inrush current to even higher levels (due to a lower room temperature resistivity) and makes the igniter prone to burnout due to unacceptably high power levels for the typical igniter geometry. These igniters are unable to radiate energy sufficiently to produce a stable temperature.
Similarly, raising the resistivity of the hot zone composition (by decreasing its MoSi.sub.2 content) has been considered as a method of decreasing the inrush current of the igniter. However, it was found that doing so not only decreased the speed of the igniter (due to a higher room temperature resistivity), it also provided an unstable igniter at high temperatures (due to its negative temperature coefficient of resistance at high temperature).
Therefore, there is a need for a ceramic igniter which has high speed but also resists cooling effects, and which has a low inrush current.